1. Field of the Invention
The present invention relates to a driving force transmission device for a four-wheel-drive vehicle based on the two-wheel drive of rear wheels enabling change between two-wheel drive and four-wheel drive, and more specifically, to a driving force transmission device for a four-wheel-drive vehicle in which the rotation of the parts not operative for the transmission of driving force in the two-wheel drive mode is stopped.
2. Description of the Related Arts
As one of driving force transmission devices for four-wheel-drive vehicles in which rear wheels are driven in the two-wheel drive mode, and the distribution control of the driving force to the front wheels is performed in the four-wheel drive mode by a multi-disc clutch mechanism, in the four-wheel-drive vehicle based on the two-wheel drive of conventional so-called on-demand type full-time rear wheels, for example, the device shown in FIG. 1 is known.
In FIG. 1, a driving force transmission device 300 is provided in a four-wheel-drive vehicle 302, and the driving force from an engine 304 are changed by a gearbox 306 and input into a driving force distribution device 308 in the driving force transmission device 300. When a multi-disc clutch mechanism 310 is released (disconnected) in the two-wheel drive mode, the driving force is transmitted to a rear wheel differential 318 through a rear wheel propeller shaft 314 as it is, and the rear wheel differential 318 absorbs the rotation speed difference between a left rear wheel 320 and a right rear wheel 322, gives a same torque to the left rear wheel 320 and the right rear wheel 322, and makes the rear wheels rotate. When the multi-disc clutch mechanism 310 in fastened (connected) in the four-wheel drive mode, the driving force is transmitted also to a front wheel differential 324 through a chain belt mechanism 312 connected to the multi-disc clutch mechanism 310 and a front wheel propeller shaft 316, and the front wheel differential 324 absorbs the rotation speed difference between a left front wheel 326 and a right front wheel 328, gives a same torque to the left front wheel 326 and the right front wheel 328, and makes the front wheels rotate. In general, in the on-demand type full-time four-wheel-drive vehicle, as the drive modes for their drivers to optionally select by switch operations, a two-wheel-drive mode, a four-wheel-drive auto mode, and a four-wheel-drive lock mode are available. The two-wheel-drive mode is the mode that is used in the two-wheel-drive state in which the multi-disc clutch mechanism 310 of the driving force transmission device 300 is released, and selected when running on dry pavement roads on which the driving force by four wheels is not necessary for the best mileage. The four-wheel-drive auto mode is the mode in which various running vehicle states are detected by sensors, and the driving force distribution of the multi-disc clutch mechanism 310 to the front and rear wheels are automatically controlled at the most suitable state by an ECU (Electronic Control Unit) based on the detection signals of the sensor, and can be always selected regardless of road surface states. In this mode, the fastening power of the multi-disc clutch mechanism 310 is increased and/or decreased continually by an actuator, and the driving force distribution to the front and rear wheels is controlled between the two-wheel-drive state in which the driving force to the front wheels is approximately zero and the maximum fastening power. The four-wheel-drive lock mode is the mode in which the fastening power of the multi-disc clutch mechanism 310 is retained at its maximum regardless of the vehicle states detected by the various sensors, and this mode is selected when to show running characteristics as the four-wheel drive on rough roads and the like at the maximum. Meanwhile, in the present application, when it is not necessary to distinguish the modes definitely, the four-wheel-drive auto mode and the four-wheel-drive lock mode are collectively called four-wheel-drive modes. However, in such a conventional driving force transmission device for four-wheel-drive vehicle, since the left front wheel 326 and the right front wheel 328 and the front wheel differential 324 are directly connected, even in the two-wheel-drive mode, the left front wheel 326 and the right front wheel 328 still rotate in the two-wheel drive mode in which the multi-disc clutch mechanism 310 is released and the driving force is not transmitted to the front wheels as shown in FIG. 1. Consequently, the respective structural elements in the front wheel driving force transmission section 330 including the front wheel differential 324, the front wheel propeller shaft 316 and the chain belt mechanism 312 rotate too, and this leads to decreased mileage by stirring resistance of the oil in this section or friction loss of bearings, which has been a problem in the prior art. Further, the multi-disc clutch mechanism 310 includes a plurality of clutch discs, and although these clutch discs are lubricated and cooled by oil, since the so-called dragging torque that occurs by the viscosity resistance of the oil arising from the rotation speed difference between the driving side (rear-wheel side) and the driven side (front-wheel side) of the clutch discs and the friction loss by the contact between the clutch discs is larger than the friction torque of the front wheel driving force transmission section 330, even if the left front wheel 326 and the right front wheel 328 and the front wheel differential 324 are disconnected, the elements in the front wheel driving force transmission section 330 are rotated by the multi-disc clutch mechanism 310, and this deteriorates the mileage, which has been another problem in the prior art. In order to decrease this dragging torque, if the supply of the oil to the multi-disc clutch mechanism 310 is stopped, or if the multi-disc clutch mechanism is used with an extremely reduced quantity of the oil, it is possible to decrease or eliminate the dragging torque that occurs by the viscosity resistance of the oil. However, there is a fear that the multi-disc clutch mechanism 310 may seize if sufficient lubrication is not provided at the time of the driving force distribution control. Furthermore, as a method to ensure sufficient intervals among the respective clutch discs in the released state of the multi-disc clutch mechanism 310, a method is suggested in which a ring spring 332 formed in a wave shape in the circumferential direction as shown in FIG. 2 is put between adjacent clutch discs of the driving side, or adjacent clutch discs of the driven side. However, according to this method, it is possible to ensure the sufficient intervals between the clutch discs of the driving side or the clutch discs of the driven side, but it is not possible to prevent the clutch discs of the driving side from contacting the clutch discs of the driven side. In the four-wheel-drive auto mode, the driving force is transmitted to the front wheels 326 and 328, and even if the transmitted driving force is at its lowest, a loss larger than that at the time of the two-wheel-drive mode occurs in the front wheel driving force transmission section 330. Consequently, the mileage becomes worse than that at the time of the two-wheel-drive mode. In this mode, the driving force distribution is automatically controlled at the most suitable state even if the driver does not perform any operation. However, in the driving condition that does not need the four-wheel drive mode, such as running on dry pavement roads, when the driver forgets the switch change to the two-wheel-drive mode, the running is continued in a poor mileage state, which has been yet another problem in the prior art. In this case, by switching between the two-wheel-drive mode and the four-wheel-drive auto mode automatically depending on driving conditions, it is possible to improve the mileage regardless of the switch operation by the driver, but for that purpose, it is necessary to quickly change between the two-wheel drive mode and the four-wheel drive mode. As another method to decrease the dragging torque of the multi-disc clutch mechanism 310, there may be a method in which the distance of the approach section shifting from the standby state at the time of the two-wheel drive mode to the fastening state at the time of the four-wheel drive mode of the multi-disc clutch mechanism 310, that is, the so-called end play, is made large, and the intervals among the respective clutch discs are ensured sufficiently in the state in which the multi-disc clutch mechanism 310 is released at the time of the two-wheel drive mode, thereby the clutch discs are prevented from contacting one another. However, when the end play is made large, it takes a lot of time in changing the mode from the two-wheel drive mode to the four-wheel drive mode, which decreases the running performance.